1. Field of the Invention
The present invention relates generally to the formation of reinforced tubing and more specifically to small diameter reinforced pipe/tubing which can be used as a catheter and the like type of medical instrument.
2. Description of the Prior Art
In the field of medical catheters, especially in the areas of small vessel therapies, there has long been a need for extremely small diameter catheters with structural properties which cannot be attained with single substance structures. The most important property missing in small diameter catheters (4Fr and smaller) has always been "torqueability" and "pushability" or the ability to accurately transmit rotational and axially acting forces from the approximate proximal end to the approximate distal end for the purposes of steering during placement and for the accomplishment of therapies. In addition, there are some requirements for burst strength under pressure which exceed the properties of single substance constructions. Even multi-polymer structures in most cases do not generate composite properties at optimum levels for many uses.
In actual fact, balloon catheters have encountered thresholds which have limited the quest for ever-smaller arrangements. This is evidenced by the fact that, although high modulus reinforced catheters have been in demand, none have successfully reached the market in sizes below 4 French due to the inability to fabricate the same accurately, in volume, and at a price affordable to the medical trade.
U.S. Pat. No. 4,764,324 which was issued on Aug. 16, 1988 in the name of Burnham (one of two inventors named on the instant application) disclosed a technique for making small diameter catheter.
However, in attempting to apply the technique disclosed in U.S. Pat. No. 4,764,324 (hereinafter Burnham '324 and which is hereby incorporated by reference thereto) to the problem of manufacturing ultra small composite tubes, it was found that desired results could not be achieved. Burnham '324 discloses the steps of: preheating the polymer substrate to a controlled degree; applying reinforcement strands/electrical conductors with appropriate winding tension to cause the strands/conductors to sink below the original substrate surface to a controlled degree; and smoothing the remaining disrupted polymeric surface to reconstruct a smooth external surface.
When attempting to only partially soften the walls of tiny tubes in the range of 0.008" down to 0.0005" in wall thickness, it was found that the heat transfer rate of the polymer to be softened is so high that the lineal separation of the heating area and the strand application area caused total fusion of the polymer. The cause of this problem was found to reside in the fact that a given amount of time is required for the heated polymer substrate to move from the heating area to the strand application point and that this was too long and allowed the fusion to go beyond the partial stage at even the highest speeds. This excessive fusion allowed the reinforcing strands to totally penetrate the tube wall and come into physical contact with the internal supporting mandrel. In other words the strands wound up for all intents and purposes, all the way through the wall they were meant to reinforce midway.